1. Field of the Invention
The present invention relates to an active matrix substrate which is disposed opposing a counter substrate with a liquid crystal layer interposed therebetween. More specifically, the present invention relates to an active matrix substrate in which transparent electrodes are arranged on a transparent base substrate, and a color filter having colored layers of red, green, and blue is formed on the transparent electrodes.
2. Description of the Related Art
A color liquid crystal display apparatus has been used as a display of, for example, a liquid crystal TV, a laptop personal computer, a note type personal computer, etc. As such a color liquid crystal display apparatus, those including a counter substrate on which a color filter is formed, an active matrix substrate on which active elements and pixel electrodes for driving a liquid crystal layer are formed, and a liquid crystal layer sealed between the substrates with a sealant are known.
The counter substrate is composed of an insulating base substrate and a color filter made of colored layers disposed at a distance on the insulating substrate. More specifically, the counter substrate includes a colored layer for each of three primary colors (red, green, and blue; hereinafter, referred to as "RGB"), and a black matrix which is disposed between the colored layers. The black matrix has a function of preventing a decrease in the contrast due to light leakage and a decrease in the display quality such as color purity. The surface of the glass substrate is smoothed with a topcoat material, and a transparent electrode is provided on the topcoat material. On the other hand, the active matrix substrate includes a glass substrate and active elements such as transparent electrodes (pixel electrodes) driving a liquid crystal layer and thin film transistors (TFTs) provided on the glass substrate.
The following five methods are generally known as a method for forming the colored layers of a color filter.
The first method is a printing method including the step of printing ink of the three primary colors, RGB, onto a glass substrate by using a printing machine.
The second method is a dispersion method which repeats the steps of coating a UV-curable resist with a pigment dispersed therein onto a glass substrate, exposing the substrate to light through a mask, and thermosetting the substrate three times with respect to RGB, thereby forming color layers.
The third method is a dyeing method including the steps of forming a resist as a dyeing preventing film on gelatin by photolithography, and dyeing the gelatin in each color of RGB with a dye.
The fourth method is an electrodeposition process including the steps of dispersing an electro-deposited polymer and a pigment, followed by performing electrodeposition coating by using electrodes formed on a substrate.
The fifth method is a micelle electrolytic process including the steps of dispersing a surfactant and a pigment, followed by electrolysis by using electrodes formed on a substrate.
In a color liquid crystal display apparatus provided with a color filter produced by the above-mentioned method, an active matrix substrate is attached to a counter substrate as described above. When the substrates are attached to each other, positioning with high precision is required so as to allow active elements on the active matrix substrate to correspond to a color filter on the counter substrate (attachment margin). Furthermore, in order to consider the attachment margin in accordance with precision for positioning and to prevent a decrease in a production efficiency caused by unsatisfactory attachment, it is required to design the black matrix so as to have a larger size. Enlargement in the size of the black matrix is one of the reasons for the decrease in the opening ratio.
In order to solve these problems, a method for forming a color filter on driving electrodes on an active matrix substrate by an electrochemical procedure has been proposed (Japanese Laid-open Publication No. 8-313726).
According to the above-mentioned proposed method, the thickness of a colored layer of each pixel is determined by the value of the current flowing through the pixel. In the presence of a supporting electrolyte which minimizes a voltage drop due to an electrolytic solution, a variation in the thickness occurs in the colored layers to be formed by a variation in the ON resistance of active elements (i.e., the resistance of the active elements when the gates are on). This variation in the thickness makes it difficult to obtain satisfactory display quality.
Furthermore, according to the above-mentioned proposed method, in the case where a color filter is provided with conductivity so as to be used as a driving electrode, a variation in the ON resistance is caused in the active elements when the resistance of an electrolytic solution is low. This causes a variation in the film thickness of the colored layers, making it difficult to obtain sufficient display quality.